The present invention relates to a carbon ribbon supply mechanism for printers, and more particularly to a carbon ribbon supply mechanism having an adjustable spindle axis effective for preventing wrinkling of thin and comparatively wide, thermal-type, carbon ribbons.
For background, reference is made to FIG. 5 which shows the main components of a conventional thermal-type printing mechanism. The printing mechanism 1 includes a thermal print head 2 having heatable dot elements for creating desired print patterns, and a platen 3 juxtaposed to the print head 2 and suitable for pressing a thermal label strip 4 and a superposed carbon ribbon 5 against the print head 2. Passage of the superposed thermal label strip 4 and carbon ribbon 5 against the print head 2 at a given speed causes the carbon ribbon 5 to be heated in accordance with the changing dot patterns of the print head so as to transfer the pattern onto labels 7 disposed atop thermal label strip 4.
Thermal label strip 4 is constituted of a backing sheet 6 which is covered with a separating agent such as silicon oil and a plurality of discreet, serially disposed, labels 7, each of which is coated with a pressure-sensitive adhesive for relatively easy detachment from the backing sheet 6. The labels 7 are separable from one another along tear-off perforations 8.
The label strip 4 and the carbon ribbon 5 are individually supplied from respective supply spindles and each is taken up by a respective take-up spindle. It is possible, if desired, to bend the backing sheet 6 sharply at a label peeling plate 9 to cause the labels 7 to separate from the backing sheet 6 after printing. The leading end of the backing sheet 6 from which the labels 7 have been detached may be taken up by a take-up spindle (illustrated in FIG. 1).
Since print head 2 heats the carbon ribbon 5 to create image patterns on the labels 7, the carbon ribbon 5 must be very thin, preferably no more than several hundredths of a millimeter. As long as the label strip 4 and the carbon ribbon 5 are relatively narrow it is easy to guide the carbon ribbon 5 without wrinkling. But that is not the case for a relatively wide carbon ribbon. Guiding and transporting a relatively wide carbon ribbon is prone to create variations in the tensile force across the width of the ribbon. These tensile force variations tend to wrinkle the ribbon. To prevent the problem, it is necessary to feed and orient the carbon ribbon 5 in a very precise manner relative to the thermal print head 2 and the platen 3. This requires the spindle from which the ribbon 5 is supplied and its guiding rollers to be precisely maintained at a parallel orientation to the printing plane of the print head and the platen.
Specifically, the angle of the supply spindle of the carbon ribbon relative to the frame of the printer must be set precisely to 90.degree.. Otherwise, variations in tensile force are certain to develop across the width of the carbon ribbon 5 with consequent wrinkling and weaving of the ribbon, as denoted, for example, by reference character W in FIG. 5. Wrinkling of the ribbon 5 adversely impacts the print quality at the location of the overlapping and wrinkling W on the ribbon 5.
The wrinkling problem can be tolerated to an extent when human readable characters are imprinted on the labels 7. However, where machine-readable characters, such as bar-codes and the like, are printed, wrinkling has a disastrous effect since the width and spacing between bars in a barcode is critical to the readability of the printed information.
The wrinkling problem does not arise if the axes of the supply spindle of carbon ribbon 5 of the platen 3 and of the take-up spindle of the carbon ribbon (all of which are mounted perpendicularly to the frame of the printer) are precisely aligned parallel to one another. But in reality it is impractical, if not impossible, to design and fabricate label printers whose internal mechanisms are or remain perfectly aligned to one another to prevent the ribbon wrinkling problem.